The present invention relates generally to an imaging method and apparatus in which phenomenon of the nuclear magnetic resonance (hereinafter referred to as NMR in abbreviation) is made use of and more particularly to an imaging method and apparatus capable of moving a part or region of the image of an object under inspection in a phase encoding direction.
Heretofore, X-ray computed tomography (CT) apparatus and ultrasonic imaging method and apparatus are extensively used as the means for inspecting or observing nondestructively internal structures of the head, abdominal region and others of the human body. Recently, attempts of conducting the similar inspection-by making use of the NMR phenomenon have achieved success, and it has turned out that information or data which could not be obtained with the X-ray CT or ultrasonic imaging method and apparatus are made available. In the inspecting apparatus operative based on the NMR phenomenon, it is required to separate and identify discriminatively the signals produced by the object under inspection in correspondence with individual parts of the object. As one of the means for realizing this function, there is known a method according to which a gradient magnetic field is applied to thereby differentiate the static magnetic fields under which the individual regions or parts of the object are placed and hence the resonance frequencies or phase encoded quantities of the individual parts to thereby obtain the positional information. The basic principle of the NMR imaging is reported in "J. Magn. Reson." Vol. 18, p. 69 (1975) and "Phys. Med. & Biol.", Vol. 25, p. 751 (1980).
As one of the imaging methods of this sort, there can be mentioned a locally selective excitation imaging method according to which a specified region or part of the human body is selectively excited to allow only that part to be imaged. In this locally selective excitation imaging, the field of view to be imaged is required to be moved. In this conjunction, a method of moving the field of view is disclosed, for example, in Japanese Patent Application Laid-Open No. 207045/1985 (JP-A-No. 60-207045). According to this method, a phase modulation and a frequency modulation is performed on the NMR signal along two orthogonal axes, respectively, in a selectively excited plane, and the orientation of the two axes is selected in dependence on the position of the field of view to be imaged so that the center position of coil coincides with the axis for the frequency modulation independent of the position of the field of view to be imaged. This method is however disadvantageous in that the orientation of the image itself varies in dependence on the position of the field of view to be imaged, not to speak of high technical expenditure involved in changing the direction of the gradient magnetic field in accordance with the position of the field of view to be imaged.